1. Field of the Invention
The present invention relates to a primary radiator used in, for example, a satellite-television reflective antenna, and, more particularly, to a primary radiator for sending and receiving circularly polarized electrical waves.
2. Description of the Related Art
A related primary radiator of this type will be described based on FIGS. 14 and 15. FIG. 14 is a sectional view of the related primary radiator, and FIG. 15 is a front view of the primary radiator viewed from a horn section. As shown in FIGS. 14 and 15, the related primary radiator comprises a circular cross-section waveguide 210 having a horn section 210a at one end thereof and having the other end formed as an enclosed surface 210b, a pair of ridges 211 formed at the inside wall surface of the waveguide 210 so as to protrude therefrom, and a probe 212 disposed between the ridges 211 and the enclosed surface 210b. 
The waveguide 210 is molded out of a metallic material, such as zinc or aluminum, by die casting. Both of the ridges 211 are integrally formed with the waveguide 210. These ridges 211 function as phase changing members (90-degree phase devices) for changing circularly polarized waves that have traveled into the waveguide 210 from the horn section 210a into linearly polarized waves. The ridges 211 have tapered portions at both ends thereof along the central axis of the waveguide 210, and have predetermined heights, widths, and lengths. As shown in FIG. 15, when a plane including the central axis of the waveguide 210 and both ridges 211 is a reference plane, the probe 212 intersects the reference plane at an angle of approximately 45 degrees, and the distance between the probe 212 and the enclosed surface 210b is equal to about xc2xc of a wavelength inside the waveguide. It is known that, instead of the ridges 211, plate members, formed of dielectric materials, may also be used as phase converting members. The dielectric plates are inserted into/secured to the inside of the waveguide 210. In that case, the probe 212 intersects at an angle of approximately 45 degrees a reference plane which is parallel to the surfaces of the dielectric plates and which passes the central axis of the waveguide 210.
In the primary radiator having such a structure, when a clockwise or a counterclockwise circularly polarized wave sent from, for example, a satellite is received, the circularly polarized wave is guided from the horn section 210a to the inside of the waveguide 210, and is converted into a linearly polarized wave when the circularly polarized wave passes the ridges 211 (or dielectric plates) inside the waveguide 210. More specifically, since the circularly polarized wave is a wave in which a combined vector of two linearly polarized waves having the same amplitudes, being perpendicular to each other, and having phase differences of 90 degrees rotates, when the circularly polarized wave passes the ridges 211 (or dielectric plates), the wave portions which have been out of phase by 90 degrees are caused to be in phase, so that the circularly polarized wave is converted into a linearly polarized wave. Therefore, when the linearly polarized wave is received as a result of coupling at the probe 212, it is possible to convert the received signal into an IF signal at a converter circuit (not shown), and to output the IF signal.
Conventionally, another known example of this type of primary radiator is a primary radiator comprising a waveguide having a horn section at one end thereof and having the other end formed as an enclosed surface, a phase converting member disposed inside the waveguide, and a probe installed between the phase converting member and the enclosed surface of the waveguide. The phase converting member converts a circularly polarized wave that has traveled into the waveguide into a linearly polarized wave. One example of the phase converting member is a dielectric plate having both longitudinal ends formed into a wedge shape. The probe intersects the phase changing member at an angle of approximately 45 degrees, and the distance between the probe and the enclosed surface of the waveguide is approximately xc2xc of a wavelength inside the waveguide.
In the primary radiator having such a general structure, a clockwise or counterclockwise circularly polarized wave transmitted from a satellite is guided to the inside of the waveguide from the horn section and is converted into a linearly polarized wave at the phase converting member. More specifically, since the circularly polarized wave is a wave in which a combined vector of two linearly polarized waves having the same amplitude, being perpendicular to each other, and having phase differences of 90 degrees rotates, when the circularly polarized wave passes the phase converting member, the wave portions which have been out of phase by 90 degrees are caused to be in phase, so that the circularly polarized wave is converted into a linearly polarized wave. Therefore, when the linearly polarized wave is received as a result of coupling at the probe, the received signal is converted into an IF signal at a converter circuit (not shown), and the IF signal is output.
However, in each of the related primary radiators constructed as described above, the waveguide is molded out of a metallic material, such as zinc or aluminum, by die casting, so that an expensive molding die having a complicated structure is required, which is a big factor in increasing production costs of the primary radiator. In recent years, to overcome this problem, an attempt to form the waveguide by winding a metallic plate into a cylindrical shape has been made in order to eliminate the use of an expensive die-casting mold. However, such a waveguide gives rise to new problems with regard to the phase converting member or members.
More specifically, in the waveguide formed by winding a metallic plate into a cylindrical shape, it is difficult to form a large protrusion on a thin metallic plate by pressing, so that, even if the protrusion is successfully formed, the protrusion have low dimensional precision. Therefore, when a ridge is used as a phase converting member, it is difficult to process. On the other hand, when a dielectric plate is used as a phase converting member, since the inner peripheral surface of the waveguide formed by winding a metallic plate is circular, it is necessary to bond the phase converting member to a predetermined location inside the waveguide while the phase converting member inserted into the waveguide is positioned with a jig at the stage of assembling the primary radiator. Therefore, the assembly work becomes very complicated.
In each of the primary radiators of this type, since the probe and the phase converting member or members intersect at an angle of approximately 45 degrees inside the waveguide, it is necessary to secure the phase converting member or members inserted into the waveguide with proper means. In general, a bonding agent is used as such means for securing the phase converting member or members. However, in the securing method using a bonding agent, it is necessary to perform the complicated step of applying the bonding agent to a joining portion of the inside wall surface of the waveguide and the phase converting member or members while the phase converting member or members are positioned with a jig. Therefore, the problem that assembly workability is poor arises. A method of securing the phase converting member or members to the inside portion of the waveguide with a screw as another securing means has been proposed. In this case, the front end portion of the screw protrudes into the waveguide, thereby giving rise to the problem of reduced performance resulting from reflection of electrical waves at the front end portion of the screw.
The present invention has been achieved in view of the problems of the related art, and has as its first object the provision of a primary radiator which has excellent assembly workability and which can be produced at a low cost. The present invention has as its second object the provision of a primary radiator whose phase converting member can be easily and reliably secured without a reduction in performance.
To these ends, according to a first aspect of the present invention, there is provided a primary radiator comprising a waveguide formed by winding a metallic plate into a cylindrical shape, a probe protruding from an inside wall surface of the waveguide in a direction of a central axis of the waveguide, and a dielectric feeder held by the waveguide. In the primary radiator, a flat portion extending parallel to the central axis of the waveguide is formed at the inside wall surface of the waveguide, and the dielectric feeder is mounted to the flat portion.
In the primary radiator having such a structure, since the waveguide is formed by winding a metallic plate into a cylindrical shape, it can be produced at a considerably reduced cost than when a waveguide formed by die casting. In addition, in the case where the dielectric feeder is mounted to the waveguide, when a portion of the dielectric feeder inserted into the waveguide is mounted to the flat portion of the metallic plate, the relative positions of the waveguide and the dielectric feeder are determined by this flat portion, so that assembly work can be simplified.
In the above-described structure, the flat portion can be formed at any location of the inside wall surface of the waveguide. However, when the structure of the first aspect is used, there may be used a first form in which the flat portion is formed at a joining portion formed by winding the metallic plate into a cylindrical shape and superimposing the end portions thereof.
When the structure of the first aspect is used, there may be used a second form in which the dielectric feeder comprises a radiator section protruding from an open end of the waveguide, an impedance converting section which becomes narrower from the radiator section towards an inside portion of the waveguide, and a plate-shaped phase converting section formed continuously with the impedance converting section, with the phase converting section intersecting the probe at an angle of approximately 45 degrees. When the structure of the second form is used, there may be used a third form in which two such flat portions are formed at two opposing locations of the waveguide on both sides of the central axis of the waveguide, and in which the phase converting section of the dielectric feeder is mounted to the flat portions. Therefore, it is possible to readily and reliably position the phase converting member and the probe relative to each other.
When the structure of the second form is used, there may be used a fourth form in which a plurality of the flat portions are formed at a plurality of locations of an inner peripheral surface of the waveguide, and in which the impedance converting section and the phase converting section of the dielectric feeder are each mounted to the flat portions, so that the dielectric feeder can be more stably mounted to the waveguide. When the structure of the fourth form is used, there may be used a fifth form in which four such flat portions are formed at four locations at an interval of approximately 90 degrees in a peripheral direction of the waveguide, so that the pair of flat portions to which the impedance converting section is mounted and the pair of flat portions to which the phase converting section is mounted are substantially orthogonal to each other. Therefore, it is possible to restrict adverse effects of each flat portion on polarized waves.
According to a second aspect of the present invention, there is provided a primary radiator comprising a waveguide including an opening at one end side, a phase converting member inserted into an inside portion of the waveguide from the opening, a plurality of retainer portions for securing the phase converting member to an inside wall surface of the waveguide, and a probe which intersects the phase converting member at an angle of approximately 45 degrees inside the waveguide. In the primary radiator, each retainer portion is separated by an interval of approximately xc2xc of a wavelength inside the waveguide in a same plane running through a central axis of the waveguide.
In the primary radiator having such a structure, since the phase converting member inserted into the waveguide is secured to the inside wall surface of the waveguide by a plurality of retainer portions, it is possible to simplify assembly work. In addition, since the interval between each retainer portion is set at approximately xc2xc of the wavelength inside the waveguide, it is possible to reduce a reflection component by cancellation of reflections of electrical waves at the corresponding retainer portions.
In the above-described structure, it is possible to use a waveguide molded out of, for example, zinc or aluminum by die casting. However, when the waveguide is formed of a metallic plate and is formed by winding the metallic plate into a cylindrical shape or a prismatic shape, it becomes unnecessary to use an expensive molding die, so that it is preferable to use such a waveguide from the viewpoint of reduced production costs of the waveguide. In this case, when a plurality of cut-up portions are formed at the inside wall surface of the metallic plate, of which the waveguide is formed, by bending portions of the metallic plate, the phase converting member can be secured to the inside wall surface of the waveguide by these cut-up portions serving as retainer portions. Alternatively, the phase converting member can be secured by using a plurality of screws as retainer portions and screwing the screws into the waveguide from mount holes formed in the waveguide.